MECHANICAL SEPARATION 299 Plate and Plate and Frame Presses.—All the usual procedures in filtering can be carried out with these devices. The leaves can be filled with material to be filtered until filtrate ceases to pass through the medium which is ordinarily canvas when the press must be discharged of its cake. Either the filtrate or cake is rejected. After the press is filled or partially filled various washes may be introduced and separately distrained if desired, the openings in the leaves being arranged to that end. Figure 19 shows a square leafed press, the usual shape of leaf, of 30 by 30-in. section outside and with center screw for binding and releasing the leaves for loading and unloading. The two great divisions of filter presses are into plate presses and plate and frame presses. The plate leaf is the supporting leaf for the filtering medium and through the openings of which the nitrate issues. The frame is the leaf into which the material to be filtered discharges. If plates only are employed they must be of the recessed type, see Fig. 20. In the recessed type of plate the depth of recess is about % in. Referring to Fig. 20 the hole in the upper left-hand corner is the feed hole. To employ the recessed type a piece of cloth is cut long enough to cover both sides. A hole is cut in the cloth where it passes over the feed hole on each side. A gommet nut is then inserted in the hole and the screw rings _ ft- _ ,. , , , ... , , FIG. 20.—Recessed plate, are tightened up on it to make a seal. When all the plates are tightened up in the press the cross-section of the plate and the filtering medium will be as represented in Fig. 21. The material to be filtered passes into the wedge-shaped spaces formed by the filter medium. The filtrate passes through the cloth and out through the cocks as indicated by the arrow points. In order to wash with recessed plates, cake must not be allowed to accumulate until it fills the wedge-shaped spaces completely, but to a degree less t than this so that the wash liquid will have a chance to pass through. The recessed-plate type of filter press should ' be employed where possible owing to its lower cost. It cannot be used with delicate filtering media such as paper because of the strain on such media at the points where it is bent. On the other hand if a very thin cake is necessary owing to the slow rate of percolation through some solids the recessed type may be used to advantage and work with relatively low pressures. With cakes from % to Yi in. the recessed type is indicated as it is impractical to make a frame so thin. The recessed type would not work advantageously with liquids which attack the filtering medium. The latter would give away under chemical attack at the points bent. As the medium lies straight in the plate and frame type of press it is better suited bo coping with fluids which attack the filtering medium. In plate-and-frame presses the leaves are assembled with the plates and frames alternately placed. Figure 22 shows the feed "eye" and part of the frame construction. Figure 23 shows the arrangement of plates, frames and filtering media in section. There is a section of cloth between each plate and frame. " Simple washing" such as has already been described can be employed with plate-and-frame PIG. 21.